Therapeutic antibodies have been used in clinical applications for over twenty years. Currently, there are fifteen anti-tumor antibody drugs in clinic, including Rituxan (1997), Herceptin (1998), Mylotarg (2000), Campath (2001), Zevalin (2002), Bexxer (2003), Avastin (2004), Erbitux (2004), Vectibix (2006); Arzerra (2009); Benlysta (2011); Yervoy (2011); Adcetris (2011); Perjeta (2012); and Kadcyla (2013). These antibodies target mainly four molecules: EGFR, Her2, CD20 and VEGF.
In general, therapeutic antibodies kill tumor cells via three mechanisms (Scott A M, Wolchok J D, Old L J. Antibody therapy of cancer. Nat Rev Cancer. (2012), 12:278-87): (1) Direct antibody action, that is, blockade or agonist activity of ligand/receptor signaling, induction of apoptosis, and delivery of drugs or cytotoxic agents. Antibody receptor activation activity can produce direct tumor cell killing effect. For example, some antibodies can bind to receptors on the surface of tumor cells, activate the receptor, leading to apoptosis (e.g., in mitochondria). Antibodies can also mediate tumor cell killing by receptor-antagonistic activity. For example, certain antibodies can bind to cell surface receptors and block dimerization, kinase activation and downstream signaling, thereby inhibiting proliferation and promote apoptosis. Binding of antibodies to an enzyme can lead to neutralization, signal abrogation, and cell death. (2) Through immune-mediated cell killing mechanisms include complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC), T cell function regulation, etc. Immune-mediated killing of tumor cells can be accomplished through the following ways: induction of phagocytosis, complement activation, antibody-dependent cell-mediated cytotoxicity, genetically modified T cells being targeted to the tumor by single-chain variable fragment (scFv), through antibody-mediated antigenic cross presentation to dendritic cell to activate T cells, inhibition of T cell inhibitory receptors, such as cytotoxic T lymphocyte-associated antigen 4 (CTLA4). Of them, the Fc portion of the antibody feature is especially important for CDC and ADCC-mediated tumor cell killing effect. (3) Specific effect of antibody on tumor vasculature and matrix, through trapping of vascular receptor antagonist or ligand to induce vascular and stromal cells ablation, including: stromal cell inhibition, delivery of toxins to stromal cells, and delivery of toxins to the vasculature. (Scott A M, Wolchok J D, Old L J. Antibody therapy of cancer. Nat Rev Cancer. 2012, 12 (4):278-87).
Therapeutic monoclonal antibody drugs have advanced anti-cancer drug research and development. However, some issues still need further study to be solved, such as antibody immunogenicity, tolerance of long-term use of tumor target, and long-term effects of simple single blockade of signal transduction pathway. In short, a simple majority of antibodies are difficult to achieve long-term efficient inhibition and killing of tumor cells.
In 1964, “Nature” magazine presented the new idea of antibody—drug conjugates (ADC) technology, which in recent years have seen breakthroughs. ADC covalently links antibody with a highly toxic drug (toxin) through a chemical linker (linker). Antibody recognizes cancer cell surface antigen molecule, the endocytosis ADC brings it into cytoplasm, and in particular intracellular environment toxins released after hydrolysis of the linker kills cells.
Seattle Genetics has developed such drug Brentuximab Vedotin (trade name Adcetris) that has been approved by the FDA to market. It is monomethyl auristatin E (MMAE), a synthetic toxic anti-cancer drug, coupled with antibody targeting lymphoma cells specific CD30 molecule, with improved efficacy of killing tumor cells.
Currently, there are more than dozens of such ADC drugs in clinical trials. Among them, Genentech and Immunogen jointly developed trastuzumab coupled with maytansines as a drug named ado-trastuzumab emtansine (Kadcyla), also known as T-DM1, to treat breast cancer. In February 2013, the FDA has approved T-DM1 for human epidermal growth factor receptor 2 (Her2)-positive metastatic breast cancer. Maytansines is a small molecule toxin that can bind tubulin and prevent formation of microtubules by forming non-reducing dual—maleimide—propanediol complex. Trastuzumab acts on breast cancer and gastric cancer by targeting human Her2. It was approved for Her2-positive cancer. However, trastuzumab cannot promote apoptosis of all of the Her2 positive cells. T-DM1 combines the selective targeting Her2 receptor trastuzumab with the potent cytotoxic agent maytansine to kill tumor cells. T-DM1 antibody binds Her2 receptors, causing cellular internalization of the maytansines released from conjugates, thereby killing the tumor cells. T-DM1 has better overall efficacy and pharmacokinetic properties and low toxicity.
Traditional small molecule chemotherapeutic drugs have strong toxicity and pharmacokinetic advantages, but in the process of treatment of tumors may affect other physiological targets with serious side effects. Antibody—drug conjugates combines targeting function and small molecule drug with particular pharmacokinetics. The structure of antibody-drug conjugates is the attachment of a monoclonal antibody with targeting function to a compound with specific pharmacological properties. This technique requires the therapeutic antibody have binding specificity to a target, to be coupled to a molecule with therapeutic effect or other functions such as cyto-toxins. Many factors affect the effect of this type of antibodies, such as endocytosis of the coupled antibody, stability of the coupling, and release and killing activity of the toxins.
Toxin molecules currently being used include tubulin inhibitors Auristatin analogues monomethyl auristatin E, monomethyl auristatin F and maytansine. Monomethyl auristatin E is a synthetic microtubule polymer inhibitor that can inhibit microtubule aggregation, interfere tumor cell mitosis and induce apoptosis (Naumovski L and Junutula J R. Glembatumumab vedotin, a conjugate of an anti-glycoprotein non-metastatic melanoma protein B mAb and monomethyl auristatin E for treatment of melanoma and breast cancer. Curr Opin Mol Ther 2003; 12 (2): 248-57. Francisco J A, Cerveny C G et al. cAC10-vcMMAE, an anti-CD30-monomethyl auristatin E conjugate with potent and selective antitumor activity. Blood 102 (4): 1458-65. Monomethyl auristatin F is an anti-mitotic Auristatin derivative with a charged phenylalanine residue at C terminus. In comparison to uncharged MMAE it minimizes damage to cell signaling pathway and minimizes cytotoxicity. A large number of test with CD30 cells found that mAb-maleimidocaproyl-valine—citrulline-p-aminobenzyloxycarbonyl-MMAF (mAb-L1-MMAF) has a toxicity that is 2,200 times stronger than MMAF only (Doronina S O et al., Enhanced activity of monomethylauristatin F through monoclonal antibody delivery: effects of linker technology on efficacy and toxicity. Bioconjug Chem, 2006; 17 (1): p114-24). Maytansine is an antimitotic agent acting as an inhibitor of tubulin polymerization, thus interfering with formation of microtubules in the cell nucleus. Maytansine also inhibits DNA, RNA, and protein synthesis, with the greatest effect being seen on DNA synthesis.
Antibodies—drug conjugates have direct and indirect anti-cancer effect. The antibody blocks or activates ligand/receptor signaling, induces apoptosis, and at the same time can present or deliver payload drug directly or indirectly (such as a drug, toxin, small interfering RNA or radioisotope) to the tumor cells. Therapeutic antibody drug conjugate utilizes dual characteristics of the antibody and the coupled drug, first is the binding function that it specifically binds to the target molecule, second is the tumor cell killing function of the antibody itself, and the third is the particular effect of the conjugated drug. Current antibody—drug conjugates drugs are limited in how to kill tumor cells directly. However, because of the tough requirement of technologies in antibody, linker molecule, toxin molecules, and conjugation, as well as the limitation of bringing toxins within the tumor microenvironment molecules, there are still some difficulties in actual clinical studies.